(HealthNewsDigest.com) - WASHINGTON - Most Americans say they support the idea of organ donation, yet fewer than half of eligible donors ever register, national polls show. That may be because supporting a good cause doesn't mean people will take action. However, people are more likely to sign up if they have positive attitudes specifically about registering as a donor, according to research published by the American Psychological Association.

In a 2005 Gallup poll, 95 percent of Americans said they "support or strongly support" organ donation, yet only 40 percent of eligible donors have registered, according to a study published online in the APA journal Health Psychology. "More than 120,000 people in the U.S. were on the waiting list for an organ transplant as of December 2013, and 18 people die each day because they didn't receive a transplantable organ," said lead researcher Jason T. Siegel, PhD, of Claremont Graduate University. "We wanted to figure out why there is such inconsistency between peoples' attitudes toward organ donation and donor registration."

If people said they felt positive specifically about signing up as an organ donor they were much more likely to register than if they simply said they strongly support the general idea of organ donation, the study found. This illustrates a psychological principle that specific attitudes are more likely to predict behavior than general attitudes, the authors wrote.

"It's a concept researchers need to keep in mind to gather more helpful data to support actions to change behaviors. Research on health issues has had a tendency to rely on global measures of attitude to predict behaviors such as smoking, drinking, taking medication or keeping doctor's appointments," Siegel said. "We should instead measure specific attitudes, whether they're about organ donations or any health-related behavior, from brushing teeth to binge-drinking."

Researchers conducted two experiments involving 516 people, none of whom were registered organ donors. In one experiment, 358 people completed an online survey; for the other, 158 college students answered a paper survey in class. Among the online group, specific attitudes as opposed to general attitudes were 75 percent (10 percent vs. 17 percent) more predictive of organ donor registration. For the students, specific attitudes were 150 percent (18.5 percent vs. 42.6 percent) more predictive of registration behavior than general attitudes. Researchers measured participants' attitudes based on their responses to questions on the surveys. All participants had an opportunity to register as an organ donor when they completed their survey. Of the online participants, 10 percent clicked on the link to a donor registration form and 13 percent of the students completed registration forms.

Both groups answered sets of questions to determine their general feelings about organ donation and different sets of questions about their specific attitudes toward becoming an organ donor. Students were asked two questions: "In general, how do you feel about organ donation?" and "How do you feel about registering yourself as an organ donor?" Their responses were rated on a continuum from negative to positive. The online participants responded that they agreed or disagreed to a range of comments, such as "I support the idea of organ donation for transplantation purposes," and they completed the sentence "Would registering yourself to be an organ donor..." with various phrases, such as "be a rewarding act?" or "be a source of anxiety?"

"Of course, positive attitudes are often not enough on their own to increase donor registration rates. Often what people need is ready access to a registration form," Siegel said.